1. Field of the Invention
The invention relates to a control system and a control method for controlling an exhaust gas sensor provided in an exhaust pipe of an internal combustion engine.
2. Description of the Related Art
Conventionally, an exhaust gas sensor is provided in an exhaust pipe, or the like, of an engine installed on a vehicle. The exhaust gas sensor measures concentrations of exhaust gas (e.g. concentrations of oxygen for fuel) that passes through the exhaust pipe, and generates a voltage signal indicative of the measured concentration. An ECU (Electronic (or Engine) Control Unit) calculates the air-fuel ratio of the exhaust gas, based on the voltage output from the exhaust gas sensor, and controls the amount of air supplied to the engine and the weight of fuel injected into the engine, so that the calculated air-fuel ratio of the exhaust gas becomes equal to a target air-fuel ratio at which the exhaust gas is cleaned up with a catalyst.
The exhaust gas sensor is made of a material such as ceramic, and incorporates a heater because the sensor is not able to detect an exhaust gas component(s) until it reaches a certain temperature or higher and becomes activated. In operation, the exhaust gas sensor is activated by being heated with the heater. When the engine that has been cooled is started, water vapor in the exhaust gas may condense and collect in the exhaust pipe. If the condensed water collects in the exhaust pipe, it may spill on the exhaust gas sensor heated by the heater, and thereby damage the exhaust gas sensor.
In view of the above-described problem, Japanese Patent Application Publication No. 2004-360563 (JP-A-2004-360563) describes a system for detecting condensed water in the exhaust pipe, the system including a water trap portion in the form of a recess formed in the exhaust pipe through which exhaust gas flows, at a location downstream of the exhaust gas sensor, for trapping and storing water in the exhaust pipe, and water detecting means for detecting water stored in the recessed water trap portion. The water detecting means has a power supply and two electrodes, through which electric current flows when water collects in the water trap portion, and an ammeter that measures current that flows between the two electrodes. When condensed water is detected in the exhaust pipe, the system takes a suitable measure, for example, stops heating the exhaust gas sensor.
In the conventional exhaust gas sensor control system as described above, the water trap portion, two electrodes, power supply and ammeter need to be additionally provided for detecting condensed water in the exhaust pipe, and the provision of these components results in an increase in the manufacturing cost.
Also, an exhaust gas sensor control system (described in, for example, Japanese Patent Application Publication No. 2007-10630 (JP-A-2007-10630) includes a heater provided in the exhaust gas sensor, operating condition storing means for storing operation conditions of the engine during the last operation of the engine, liquid water presence determining means for determining whether condensed water from exhaust gas is present in the exhaust pipe when the engine is started this time, based on the stored operating conditions in the last engine operation, and heater control means for controlling preheating by energizing the heater when there is no condensed water.
However, the conventional exhaust gas sensor control system may encounter a situation where water droplets, formed from water contained in exhaust gas flowing from the engine into the exhaust gas sensor, condense on the exhaust gas sensor if the interior of the exhaust pipe is not dry, even if there is no condensed water of exhaust gas in the exhaust pipe. In this case, the exhaust gas sensor may be damaged if the sensor is rapidly heated. Also, if the exhaust pipe has a special structure that causes the temperature of exhaust gas to decrease by the time the exhaust gas reaches the exhaust gas sensor, for example, if there is a long distance from the engine to the exhaust gas sensor, or a component, such as a catalyst, is disposed between the engine and the exhaust gas sensor, water droplets are likely to condense on the exhaust gas sensor.